a) Field of the Invention
This invention relates to a brake band suitable for use in an automatic transmission for an automotive vehicle or the like, and particularly to a brake band allowing the automatic transmission to exhibit stable speed changing performance over a prolonged period of time.
b) Description of the Related Art
To facilitate understanding of the present invention, the basic construction of a brake band will be described with reference to FIG. 4. A brake band 10 is formed of a strap 3 made of a thin steel plate or the like and a lining 2 bonded on an inner side of the strap 3. In use, the brake band 10 is arranged surrounding a drum 1.
Numeral 4 designates a bracket on a side where braking pressure (hydraulic pressure P.sub.1 in FIG. 4) is applied (hereinafter called "the apply side"), whereas numeral 4' indicates a bracket on a fixed side (hereinafter called "the anchor side"). Designated at numeral 5 is a hydraulic device.
When the drum 1 is rotating from the anchor side toward the apply side (i.e., in a direction indicated by arrow R.sub.1), the direction of rotation of the drum is called "the energizing direction". An opposite case (the rotating direction indicated by arrow R.sub.2), on the other hand, is called "the deenergizing direction".
Before use, a lining of a brake band has a small percent contact area and also a small coefficient of friction. In the course of use, however, the lining gradually wears out on the apply side, resulting in the problem that the percent contact area and the coefficient of friction both increase and significant variations take place in speed changing performance.
FIG. 5 diagrammatically illustrates variations in the coefficient of friction, .mu. and percent contact area, P of a conventional brake band as a function of the number of applications (cycles), n plotted along the abscissa. In the diagram, the solid curve M shows variations in the coefficient of friction while the dashed curve S depicts variations in the percent contact area. It is indicated that both the coefficient of friction and the percent contact area increased to values approximately four times as much as their values at the time of the commencement of use of the brake band until n reached about 6,000 cycles. In the diagram, the left-hand ends of the respective curves show the corresponding values at the time of the commencement of use of the brake band while their right-hand ends indicate the corresponding values when the brake band had been applied 6,000 times.
In FIG. 2, coefficients of friction, .mu. are plotted along the ordinate whereas percent contact areas, P are plotted along the abscissas. FIG. 2 indicates that the friction of coefficient proportionally increases with the percent contact area.
The percent contact area and coefficient of friction of a conventional brake band both increase with the number of applications as described above, so that the speed changing performance does not remain constant.
FIG. 3 illustrates how to calculate a percent contact area. A line is drawn at a height 5 .mu.m the way down from the top of a tallest asperity on a surface of a brake lining. Lengths l.sub.1, l.sub.2, l.sub.3, l.sub.4 of individual asperities cut by the line are added. The percent contact area is expressed by the percentage of the sum of the lengths based on an overall length L, namely, (l.sub.1 +l.sub.2 +l.sub.3 +l.sub.4)/L.times.100.
The percent contact area of a brake band can be represented generally by the following formula: ##EQU1## where L: the overall length of the brake band; and
n: the number of asperities.